A compressor is a machine which compresses a gas medium (e.g., air, nitrogen gas, oxygen gas, hydrogen gas, ammonia gas, carbon dioxide gas, carbon monoxide gas, or hydrocarbon gas) by means of external work and which feeds pressure-elevated compressed gas. Compressors are divided into a positive displacement compressor and a rotodynamic compressor, on the basis of the operational mechanism of elevating air or gas pressure. The positive displacement compressor is classified into a reciprocating-type compressor and a rotary-type compressor.
As compared with conventional reciprocating-type compressors, rotary-type compressors have been more widely used, from the viewpoints of resource saving, reduction of noise and vibration, efficiency, etc.
In rotary-type compressors, lubricating oil is in contact with high-temperature and high-pressure air or gas. That is, rotary-type compressors are operated under more severe conditions as compared with reciprocating-type compressors. Therefore, a compressor oil employed in rotary-type compressors must have higher thermal/oxidation stability.
Meanwhile, rotary compressors have been more and more downsized recently, and are often operated under severe conditions; for example, in an oxidizing gas such as SOX or NOX or a cutting mist atmosphere. In such a case, sludge is formed in lube oil, which is immediately deposited on an inner portion of the compressors or causes clogging of a filter, in some cases resulting in operation failure.
Under such circumstances, there is demand for a lube oil having high resistance to sludge formation.
Patent Document 1 discloses a lubricating oil composition essentially containing N-p-alkylphenyl-α-naphthylamine having a branched alkyl group derived from propylene oligomer, and p,p′-dialkyldiphenylamine having a branched alkyl group derived from propylene oligomer.
Patent Document 2 discloses a lubricating oil composition containing N-p-alkylphenyl-α-naphthylamine and p,p′-dialkyldiphenylamine in specific amounts with a specific ratio by weight.
Patent Document 3 discloses a lubricating oil composition containing 2-tert-butyl-4-alkyloxymethyl-6-alkylphenol (a phenol-based antioxidant), N-p-alkylphenyl-α-naphthylamine, and p,p′-dialkyldiphenylamine.
Patent Document 4 discloses a lubricating oil composition containing a phosphorus-containing phenol-based antioxidant, a phosphorus-free phenol-based antioxidant, and an amine antioxidant.
Patent Document 5 discloses a lubricating oil composition containing phenyl-α-naphthylamine, p,p′-dialkyldiphenylamine, and a phosphorus-containing extreme-pressure agent.
However, those lubricating oil compositions disclosed in the Patent Documents cannot simultaneously attain high levels of thermal/oxidation stability, resistance to sludge formation, lubricity, long service life, or water separation for use as compressor oil. Therefore, further improvement has been demanded for the performance of the lubricating oil compositions.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 3-95297    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 7-252489    Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 9-296192    Patent Document 4: Japanese Patent Application Laid-Open (kokai) No. 11-35962    Patent Document 5: Japanese Patent Application Laid-Open (kokai) No. 2005-239897